There is an increasing demand for the ability to detect magnetic fields that vary on a small spatial length scale of d μm, where d may vary from the order of 0.01 microns (10 nm) to the order of 10 microns. As one particular example, fields that vary on such a scale are associated with the magnetic moment of magnetic particles (MPs) of diameter d. One method for detecting the presence of such a particle is to sense the accompanying, local magnetic field. These particles, in turn, may be used for a variety of applications, such as “tagging” biological or chemical targets. Thus, sensing the presence and/or location of magnetic “tags” permits the identification of the associated target. The background art essentially involves two categories, the use of individual sensors to detect local magnetic fields, and common gradiometric techniques used for measuring field and field gradients that vary on a macroscopic scale.
Regarding the first category, the prior art describes the use of individual field sensors for the specific application of detecting magnetic particles. Criteria for suitably designed sensors are known to those skilled in the art. It is generally desirable to fabricate a magnetic field sensor that can be located in close proximity to the particle, within a length scale of order d, and furthermore the size scale of the sensor itself must be of order d. U.S. Pat. No. 5,981,297, Baselt, describes a scheme using magnetoresistive sensors for the specific application of detecting magnetic particles that are used to tag biological agents. U.S. patent application Ser. No. 09/497,754, filed Feb. 4, 2000, by G. A. Prinz and M. M. Miller, describes a specific type of sensor configuration and teaches critical details for realizing such a sensor. In both these disclosures, Giant Magnetoresistance (GMR) sensors are a preferred embodiment. The advantage of GMR is its relatively large magnetoresistive effect and the associated high sensitivity that, in general, surpasses that of Hall effect devices. However, the sensitivity of a GMR element is compromised when the device is fabricated on a small scale because of magnetostatics. For example, a sensor that is many microns in size may be capable of detecting magnetic fields that are uniform over the sensor in the nT range. However, the fringing field from a MP that is locally highly non-uniform becomes problematic for several reasons. First, if the active area of the sensor is much larger than the MP then the magnetoresistance induced from the fringing field is largely shunted out by the unperturbed portion of the sensor. Furthermore, if the MP is extremely small the highly localized fringing field will be unable to affect a comparably small portion of the sensor. This is because the magnetic moments in the sensor cannot be arbitrarily rotated at a greater spatial variation than that allowed by the exchange interaction. Second, when the GMR sensor has dimensions on the order of d the micromagnetic configuration of the sensor becomes the dominant concern as d becomes very small. Any conceivable GMR sensor element will have to overcome magnetostatic demagnetization barriers in either the “quiescent” state or when the magnetic moments are either reversing or rotating in response to the MP. For many reasons, fabrication of GMR devices become extremely difficult for length scales d less than 1 micron.
Regarding the second category, techniques of gradiometry, using devices called gradiometers, are well known and used for measurements of fields and field gradients on macroscopic length scales of order 1 cm and larger. On a smaller length scale, several gradiometers designed for use as read heads for reading out bits of data stored on magnetic media have been described in the literature. These designs do not use sensors that are integrated on a common chip. This application typically involves a sensor fabricated on the head of a mechanical servo arm with tracks of magnetic bits moving below the arm. These read head gradiometers have not proven to be commercially successful.
In general terms, a gradiometer is a magnetic field measurement system composed of n subsystems such that different linear combinations of the n sensed values correspond to measurements of magnetic fields that are spatially invariant (zeroeth mathematical moment), have a first order gradient (first moment), second order gradient (second moment), etc. A simple gradiometer with two such subsystems or components, A and B, can obtain a measurement discriminating between magnetic fields that are either spatially constant or varying. Such a two component system describes the vast majority of available gradiometers. Each component A or B can be formed of any of a variety of common magnetic field sensors, such as inductive loops, magnetoresistors or Hall plates, and the associated electronics. Because the spatial separation of components A and B determines the length scale over which a field gradient is measured, the expected length scale of a given measurement often determines the selection of sensor. For example, a pair of inductive coils that is wound with diameter of 2 mm, a thickness of 0.5 mm for each coil and a separation of 1 mm between coils is appropriate for measuring fields with gradients that vary on a length scale of order 1 mm. In any specific application of a gradiometer, values of voltage measurements VA and VB taken from subsystems A and B, respectively, are used to form linear combinations V0,1=VA+/−VB, where voltages V0 and V1 represent measurements of spatially constant and nonconstant magnetic fields, respectively, and where the relative signs of VA and VB depend on their configuration such that subscripts 0 and 1 may be interchanged accordingly.
Accordingly, a gradiometer is typically composed of two elements that are capable of sensing a magnetic field, in a geometry where magnetic fields that vary on a long spatial scale are nulled but fields that vary on a short spatial scale are measured. For the integrated gradiometer, several different kinds of devices are available for the sensing of magnetic fields. Examples that operate at room temperature include inductive coils and a variety of magnetoresistors, including anisotropic magnetoresistive elements, spin valves and magnetic tunnel junctions. A third family of devices is one of the oldest commercial field sensors, the Hall plate. The family of Hall devices includes dozens, if not hundreds, of patented variations [see, for example, R. S. Popovic, “Hall-effect Devices,” Sens. Actuators 17, 39 (1989); R. S. Popovic, “Hall Effect Devices” (Adam Hilger, Bristol, 1991)].
A “double layer Hall sensor array” cryogenic device has been reported in the literature [Y. Abulafia et al, APL 72, 2891 (1998)], where the device was used to measure two magnetic field components (in-plane, or x, and perpendicular-to-plane, or z) associated with macroscopic superconducting samples. The device incorporated a series array of 5 structures, each composed of two vertically stacked Hall crosses with transverse dimensions of 10 μm, where each of the two layers was a two dimensional electron gas (2DEG) designed to operate at cryogenic temperatures, and the two “layers” were separated by an insulating layer with a thickness of 1′˜1 μm. A superconducting sample with dimensions of order 1 mm was placed over the array. Hall measurements from each pair of sensors were individually recorded. They were used to calculate the gradient of the z component at the five x-positions that corresponded to the five pairs, and the x component of field was then calculated. The x- and z-components of magnetic field at the surface of the sensor array were reported. The z-component of field associated with the macroscopic sample had a gradient that varied on a spatial scale of about 10 microns. The device has several shortcomings. It operates at cryogenic temperatures. It is not capable of measuring the field gradients, that vary on a spatial scale less than 10 microns, associated with magnetic particles because the transverse dimensions are too large. The device does not operate using a comparison mode, whereby the voltages of both sensors of a pair are automatically summed or subtracted giving an output in proportion to the field and field gradient, and the device is not used along with an application of a constant external field (all measurements are in zero external field).